188 MICHIGAN SURVEY, 1908. 



in the area should carry more weight than abundance of individuals. 

 But by the proper correlation of criteria, such cases will not be con- 

 fusing. This sort of differentiation is well shown among beetles by 

 Wickham, '04, '05. 



This criterion evidently does not apply, at least in part, to the fauna 

 now found in glaciated North America. This is made probable through 

 origin elsewhere and a later expansion in the glaciated area as the lee 

 Age declined. 



11. Ecological differentiation. Ecological and taxonomic dififerentia- 

 tibn need separate recognition, although they are frequently not dis- 

 tinct because of their intimate genetic relations. With great taxonomic 

 diversity, within a group, there is almost certain to be ecologic diver- 

 sity; but generally much less attention is given to the ecological diver- 

 sity. Compare No. 1. 



The following outline of the centers of beetle dispersion must be 

 considered provisional and suggestive, as it is a subject which has 

 received but little attention from the standpoint here presented. The 

 preceding criteria, of taxonomic and ecologic nature, have been given 

 much emphasis in locating the present centers of dispersal. Through- 

 out this section references are given to significant papers, but this 

 does not imply that the views here presented' are approved by the 

 authors to whom reference is made. These references also apply to 

 the quotations already given in detail. 



1. The American Tropical Center. From Panama northward to the 

 Mexican plateau is the main body of the tropical center. Narrow 

 elongations extend coastwise on each side of the plateau, and on the Gulf 

 Coast to the Rio Grande river. William Wickham '97a; Schwarz '01; 

 Townsend '95, '97; Tower '06. Outlying colonies are found on the 

 Pacific coast of Mexico and at the Southern extremity of Lower Cali- 

 fornia; at the mouth of the Colorado river (Schwarz) ; and in south- 

 ern Florida (LeConte '78a; Schwarz '78, '88). The fauna of the West 

 Indian Archipelago probably belongs with this great composite center. 



A vast number of beetles aire characteristic of this complex area. The 

 Biol^gia Centrali-Americana devotes thirteen volumes to the descrip- 

 tion of beetles from part of this area. Within the United States the 

 attenuated tropical element has been most carefully studied and its 

 faunal aflfinities determined by Schwars^ ('88) and Wickham (97a). 



This tropical center is composed of several distinct units. This is 

 an ancient center of origin, preservation and of dispersal. The routes 

 of dispersal into the United States have been along both coasts of 

 Mexico and via the West Indies. It was practically uninfluenced by the 

 Ice Age. 



2. The Mexican Plaleau and the Southioestern Dry Desert. This center 

 includes the Mexican Plateau ; most of Lower California ; the deserts of 

 southwestern United States; the low lands of California; the Great 

 Basin and the Great Plains northward into Canada and east to the 

 forests. LeConte '51, '59, '60. '02 ; VanDyke '01 ; Wickham '96, '98, '04, 

 '05; Tower '06; Fall and Cockerell '07. 



Characterized by numerous desert species; wingless Tenebrionidae 

 (Horn '71), and Cicindellidae. A given locality is characterized by a 



